It's Xi's first visit to North Korea in seven years.
Earlier on Monday, he was given a lavish welcome upon arrival at Pyongyang's international airport, as he and his wife Peng Liyuan were greeted by Kim and his wife Ri Sol-ju.
Xi later arrived at Pyongyang's main square, where a military honour guard and thousands of people, including children carrying balloons and hopping, staged a welcoming ceremony.
In a summit later on Monday, Xi expressed China's willingness to expand co-operation in a wide range of areas including trade, agriculture, construction and technology, China's state broadcaster CCTV said in an online report.
Xi said the two countries should strengthen strategic co-operation and firmly safeguard their respective sovereignty and security interests, according to the report.
Kim said Xi's visit "clearly demonstrates how unbreakable" the North Korean-China relationship was, CCTV said.
It cited Kim as saying that consolidating a new era of friendship between the two countries was the "unchanging strategic choice" of North Korea.
Full details of the meeting were not available.
But foreign experts earlier predicted the meeting would have big ramifications on bilateral ties and beyond, as they both seek to fully restore their traditional alliance in the face of separate confrontations with the US.
Xi and Kim last met in Beijing in September after viewing a military parade alongside Russian President Vladimir Putin and other foreign leaders.
Xi's trip comes after his back-to-back summits with US President Donald Trump and Putin in Beijing in May.
Xi is expected to meet Trump again on a planned US visit in September.
Xi will try to demonstrate China's "sway over the Korean Peninsula" and "a leadership role in entire Northeast Asia in the age of strategic competition with the US," said Kwak Gil-sup, the head of One Korea Center, a website specialising in North Korea affairs.
China has long been North Korea's main diplomatic backer and economic lifeline of its impoverished neighbour.
But there have been questions about their ties in recent years, with North Korea prioritising co-operation with Russia by supplying troops and weapons to support its war against Ukraine.
In return, North Korea has received economic and military assistance from Russia.
Restoring an exclusive influence over North Korea would give Xi a leverage in dealings with Trump, who has repeatedly expressed his desire to restart diplomacy with Kim, experts say.
